Rapid advances in DNA sequencing technologies have allowed comprehensive analysis of patient samples for disease-associated mutations. A major complicating factor is that mutations are often found in small pedigrees or sporadic cases of disease making it difficult to build a strong genetic argument for pathogenicity. In these cases functional analyses are critical for evaluating the pathogenicity of rare variants. Furthermore, functional evaluation can provide insight into the molecular mechanism of disease as well as platforms for developing therapeutics. The Undiagnosed Diseases Program (UDP) is cataloging promising rare variants identified in patients with myriad clinical phenotypes. A subset of these variants reside in genes encoding aminoacyl-tRNA synthetases (ARSs), which are critical enzymes involved in charging tRNA with cognate amino acids. The UDP has identified six ARS variants-two each in glycyl-tRNA synthetase (GARS), alanyl-tRNA synthetase (AARS), and aspartyl-tRNA synthetase (DARS). Our laboratory specializes in evaluating the pathogenicity of ARS variants in vitro and in vivo. Indeed, we identified the first disease-associated ARS mutations in the GARS gene in 2003 and have been continuously studying this class of genes ever since. We propose two specific aims to evaluate the pathogenicity of the six ARS variants and to understand how they cause human genetic disease. First, we will test each ARS variant for a loss-of-function effect in biochemical, yeast complementation, and cellular localization assays. Second, we will determine if each ARS variant is able to exert a dominant, toxic phenotype in zebrafish, similar to other disease-associated ARS mutations. These efforts compose a thorough evaluation of the pathogenic potential of each ARS variant and will provide the basis for developing therapies for affected individuals.